At first glance, Mercury appears to be no different
than our moon. It is a dry, nearly airless
world that is heavily cratered, and it is only 40% larger than our moon.
The planet has not been resurfaced since its formation, and the lack
of any erosional processes preserves the craters, much like our moon.

Because of its atmospheric absence, the temperatures
of the side facing the sun versus that facing
away from the sun are very different: The side towards the sun is a sizzling
350 °C (662 °F), while the night side is a cold -180 °C (-292 °F).
The reason for this huge difference is that it is the atmosphere (and
water in the case of Earth) of planets that
moves the heat around, allows night and day temperatures to remain relatively
constant.

But this is where the resemblance with our moon stops.
Mercury has no large mare regions (dark "seas"), it has a significant
internal magnetic field, and there is evidence that it has shrunk in
size since it formed. Mercury is, in many ways, one of the least-understood
planets in our solar system, since it was only visited by one spacecraft
in the 1970s. It is hoped that the MESSENGER craft, now en route, will
provide many clues to help decipher Mercury's complex history once the
craft arrives in 2011.

Atmosphere

As can be seen in the planetary data table below, Mercury's
atmosphere is essentially a vacuum. It exerts a pressure of only approximately
10-15 bars. If you drop a feather on your hand, it exerts
a pressure of about 10-3 bars, which is one trillion times
more than the atmospheric pressure on Mercury.

Geology

Even though it is a "dead" planet and only
45% of it has been photographed, Mercury has a varied geology. The Mariner
10 images provided a resolution of up to 2-3 km/px (1-2 miles/px). Most
of Mercury consists of heavily cratered terrain. In general, Mercury
has less craters on it per unit area than our moon. This could be for
several reasons, but one of the most likely is that the intercrater plains
cover up many craters that would otherwise be visible.

One of the most prominent features of Mercury is the
Caloris Basin, which is an impact crater about 1300 km in diameter (about
25% of Mercury's size). It is believed that Caloris Basin formed several
billion years ago. Its ejecta has been mapped across a fair portion of
the planet, and the basin itself has a varied geology. It has terraced
walls, consistent with most large impact basins in the solar system.
Caloris Basin represents a significant event in Mercury's history.

Another important aspect of Mercury's geography is a
scarp - a ridge - that lies along most of the photographed equator of
Mercury. The scarp's height - on the order of 1 km - indicates that Mercury
probably contracted during its history. The contraction would have been
a result of the planet cooling as a whole; the height of the scarp indicates
Mercury probably shrunk by about 1-2 km in diameter.

Other Features

Mercury is the smallest non-controversial planet in
the solar system (Pluto is not considered by
most astronomers to be a planet). Therefore, it has very weak gravity.
It also has nearly no atmosphere. This is believed to be a result of
its weak gravity. However, Mercury might have been hit by an object the
size of our moon early in its history. This could have stripped off most
of its atmosphere and crust, which could be why it has an enormous core:
Mercury's core is 75% of Mercury by volume and 80% of it by mass.

This giant iron core is probably the source of Mercury's
intrinsic magnetic field. Its field is only about 1% of Earth's but it
is still significant that an otherwise geologically dead planet has a
magnetic field at all.

Its proximity to the Sun has resulted in two interesting
characteristics of its orbit. First, it is locked into a 3:2 resonance
with the Sun. This means that for every two Mercurian years, it has exactly
3 days, where the day is measured with respect to the stars (sidereal
day). Because of its slow rotation rate on its axis, the length of time
between sunrise and the next sunrise is nearly 176 Earth days.

The other orbital feature is that since it is so close
to the Sun, general relativity plays
a small role in governing its shape. None of the planets always return
to exactly the same location each orbit, as is predicted from Newtonian
mechanics, but in Mercury's case, the orbit traces a significant
rosette pattern in space. Mercury's perihelion processes at a rate of
574 arcsec per century. In the 1800's, French astronomer Urbain Le Verrier
accounted for every classical perturbation (from other solar system bodies
tugging on it), but there were still 43 arcsec that were unaccounted
for. This extra 43 arcsec is exactly what is predicted with general relativity.

Mythology and Naming Schemes

Mercury is named for the ancient Roman god of messengers
and merchants who was also the swiftest of the gods. Jupiter gave him
a pair of winged sandals, with which he could fly faster than the wind.
This is why the planet is named for him: It has the fastest year, circling
the Sun in just 88 days, and it also moves the fastest of all the planets
at nearly 48 km/s (30 miles per sec).

Craters on Mercury are named after dead artists from
every culture across the globe in the fields of music, painting, and
authors, who have made outstanding or fundamental contributions to their
field.

Ridges (dorsum singular, dorsa plural) are named after
dead scientists who have contributed to the study of Mercury.

Plains (planitia) are named after other messenger deities
in other cultures, such as Japanese, Norse, and Hindi, or after other
names for the planet Mercury in various languages.

Scarps (rupes) are names after famous ships of discovery
or scientific expeditions.